//Given two integers n and k, return all possible combinations of k numbers out 
//of the range [1, n]. 
//
// You may return the answer in any order. 
//
// 
// Example 1: 
//
// 
//Input: n = 4, k = 2
//Output:
//[
//  [2,4],
//  [3,4],
//  [2,3],
//  [1,2],
//  [1,3],
//  [1,4],
//]
// 
//
// Example 2: 
//
// 
//Input: n = 1, k = 1
//Output: [[1]]
// 
//
// 
// Constraints: 
//
// 
// 1 <= n <= 20 
// 1 <= k <= n 
// 
// Related Topics 回溯算法 
// 👍 593 👎 0


package leetcode.editor.cn;

import java.util.ArrayList;
import java.util.List;

//Java：Combinations
class P77Combinations {
    public static void main(String[] args) {
        Solution solution = new P77Combinations().new Solution();
        // TO TEST
        System.out.println(solution.combine(1, 1));
    }

    //leetcode submit region begin(Prohibit modification and deletion)
    class Solution {
        List<List<Integer>> res = new ArrayList<>();

        public List<List<Integer>> combine(int n, int k) {
            int[] arr = new int[n];
            for (int i = 0; i < n; i++) {
                arr[i] = i + 1;
            }
            combine(arr, 0, k, new ArrayList<>());
            return res;
        }

        private void combine(int[] arr, int start, int k, ArrayList<Integer> list) {
            if (k == 0) {
                res.add(new ArrayList<>(list));
            }
            for (int i = start; i < arr.length; i++) {
                list.add(arr[i]);
                combine(arr, i + 1, k - 1, list);
                list.remove(list.size() - 1);
            }
        }
    }
//leetcode submit region end(Prohibit modification and deletion)

}